The objectives of the proposed project are: 1. To use the technique of Energy Dispersive X-ray Spectroscopy (EDS) to quantify calcium and other biologically important elements in subcellular regions of mammalian myocardium known to bind or sequester calcium (terminal cisternae, mitochondria, I-band). 2. To measure changes in the concentrations of calcium and other elements at these sites under conditions which have been hypothesized to directly affect storage or release of calcium in myocardium. In particular, we will investigate the effects of a) caffeine, b) varying extracellular calcium, and c) varying sarcomere length on resting and tetanized (in the presence of caffeine) ventricular muscle. 3. To adapt a relatively new freezing method (propane jet freezing) for the study of intact myocardial tissue, with the ultimate goal of studying beat-to-beat variations in subcellular elemental distributions.